Under the Laws of Intestacy your wife/husband will inherit the whole estate.
Writing a Will lets your loved ones and others know you cared enough to sort things out in advance.
No opportunity to make other gifts or arrangements
Will your wife/husband have access to immediate funds?
What about your funeral ? No wishes expressed, maybe no financial provision
Your estate will be handled by a person you did not choose and a professional will charge your estate for costs and expenses.
Your estate will almost certainly take much longer to conclude
In a Will you can make provision for a situation if both of you die
If you have children, a Will allows you to choose a guardian for any children under 18, otherwise the Local Authority will decide what happens to your children. This is a lengthy and difficult process for everyone.
If you have children, they will get nothing.